In general, the use of USB cables to provide data connections between electronic devices has increased in popularity over the last decade. Today, USB cables are commonly used to connect devices, including smart phones, cameras, keyboards, mice, etc., to, for example, a desktop or laptop personal computer. In addition to data transmission, older versions of the USB specification provided for low-level power delivery that was only sufficient for charging small electronic devices. For example, the USB 2.0 specification provided for delivery of two and a half watts (2.5 W) of power, enough to charge a portable music player. The USB 3.0 specification provided for delivery of four and a half watts (4.5 W) of power, enough to charge a cell phone. The USB Battery Charging 1.2 specification provided for delivery of seven and a half watts (7.5 W) of power, enough power to charge a digital camera, but prohibited data transmission when delivering power at that level.
A USB Power Delivery specification was first released in 2012, and version 2.0 (V2.0) of the USB Power Deliver specification was released in 2014. FIG. 1 schematically depicts the architecture of a USB power delivery (PD) connection. The Power Delivery specification provides for delivery of up to one hundred watts (100 W) of power via USB cables while simultaneously transmitting data. In particular, the Power Delivery specification includes a profile—one of multiple profiles—for supply of twenty volts (20V) of voltage and five amperes (5 A) of current via a USB cable. This profile makes it possible to power laptop computers and disk drives using a USB cable. Additionally, simplified connections between devices are possible. For example, a laptop computer that was previously connected to a display device such as a flat-panel TV, e.g., by an HDMI cable, can now also be charged by the TV—which is itself powered through an AC wall outlet—without increasing the number of cables (e.g., by replacing the HDMI cable with a USB PD connection). A single USB cable can provide the necessary power and data connection between the TV and the laptop.
One advantage of the USB Power Delivery specification is that it permits power flow in both directions. As such, a connected device can act as a power source to the device at the other end of the cable and/or can act as a power sink for the other device. The two devices are able to negotiate their sink/source roles as well as the voltage and current requirements over the USB cable. In a USB cable with Type-A or Type-B connector, this negotiation is done over the same wire that delivers the power—the VBUS wire—using frequency shift key (FSK) modulation.
The recent USB 3.1 specification defines a new Type-C connector. Unlike the Type-A and Type-B plugs, the Type-C plug does not have to be oriented in particular top-bottom configuration to mate with the socket. USB cables that have Type-C connectors have a separate wire known as the configuration channel (CC) that is used—instead of VBUS—for the negotiation of power-requirements between connected devices.
The availability of true power delivery over USB cables makes longer runs of cable more desirable. Currently, a typical USB cable is three to ten feet in length. However, with the USB PD specification, it may now be desirable to connect a display such as a flat-panel TV to a laptop computer that is operated by a user sitting on a couch at the opposite end of the room from a display. Similarly, the display may be connected to a disk drive that is located in a discrete location in another part of the room. As such, longer cable lengths for USB cables become more desirable. For example, for a particular application a 100-foot cable may be desirable.
With longer cable runs, the voltage drop over the distance of the cable presents a problem, especially at higher currents. FIG. 2 schematically shows a USB cable connected to a power consuming device. The cable contains two power wires—VBUS and Gnd—connected to both the power supply and the consumer device to create a circuit loop. If the gauge of the wires are AWG#22 wires (dia. 0.644 mm) and each wire is one hundred feet long (100 ft.), then each wire will have a resistance of approximately one and six tenths ohms (1.6Ω). If the power supply is twenty volts (20V) and the current draw of the system at twenty volts (20V) is five amperes (5 A), then the voltage drop across each wire is eight volts (8V) according to Ohm's law (5 A*1.6Ω=8V). The combined voltage drop across the cable is sixteen volts (16V) (e.g., 2*8V=16V). This only allows for a four volt (4V) voltage drop across the device, which is below the voltage rating for most electronic devices. If the gauge of the wires are AWG#18 wires (dia. 1.024 mm), then each one hundred foot (100 ft.) wire will have a resistance of approximately six tenths ohms (0.60Ω). In that case, for a twenty volt (20V) power supply and five amperes (5 A) of current, the combined voltage drop across both wires in the cable would be approximately six volts (6V). This allows for a fourteen volt (14V) voltage drop across the consumer device, which is enough for some but not all consumer applications. For example, more than fourteen volts (14V) is generally required to charge a laptop battery.
Another power delivery profile supported by the USB Power Delivery specification is ten watts (10 W) of power at five volts (5V) and two amperes (2 A). For this profile, a one hundred foot (100 ft.) USB cable using two AWG#22 gauge power wires requires approximately a six and four tenths volt (6.4V) voltage drop across its length (2*2 A*1.6Ω=6.4V), which means it cannot work with a five volt (5V) power source. A one hundred foot (100 ft.) USB cable using two AWG#18 gauge power wires would require approximately a two and four tenths volt (2.4V) voltage drop across its length, meaning that only two and six tenths volts (2.6V) is available at the consumer device. This amount of voltage is insufficient for most consumer electrical devices.